L-Carnitine is a supplement found in muscle and organ meats. Its main function is in metabolising  fatty acids to produce energy in the mitichondria (main body of cells). It seems that the more carnitine you have the more effectively you will produce energy from bodyfat, leading to more bodyfat being burnt per day/bout of exercise.  It was never made clear or fully tested that an excess of carnitine (past what we produce with a healthy intake of meats) would lead to increased fat burning. I would predict that alot of us with moderm diets would be marginally defficient in carnitine, but there are more effective things you can spend money on. One demographic of people who would benefit from Carnitine are vegetarians due to their non existant intake of lean meats. For fat burning, it is generally believed 1-3 gms/day spread out evenly is sufficient.